Preparing to attack the d5-pawn and maybe play Nf8-e6, exploiting the pin. There are alternatives.
16. c1f4  16... f8e8 This type of position is sometimes seen in the Grunfeld Defense. White has the pair of bishops and a strong passed pawn, but Black is not without chances with a queenside pawn majority and the idea of blockading the pawn with Ne8-d6 or attacking it with Rad8. 17. d1d2  17... a8d8  18. a3c2 Both sides play natural moves. The knight returns to a better position.